If New Yorkers have learned one thing about Mike Bloomberg these past few years, it’s that “voluntary” hasn’t been a key feature of his signature initiatives.

In his time in office, the mayor has imposed bans on smoking, trans fats and the sale of large sugary drinks (later overturned). He has also targeted Styrofoam, cigarettes and salt. Now comes word about a new initiative for urban composting.

The claim is that city residents will begin composting “voluntarily.” And we all know what that means. It means the goal is for the program to become mandatory.

As usual, the claimed benefits are large. By separating out the food waste — about a third of what residents put in our trash cans — the city would supposedly save $100 million a year from what it now costs to send it to landfills in Ohio, Pennsylvania and South Carolina. The idea is that we would put our food waste out on the curb for pick-up, to be composted into fertilizer or converted to natural gas.

We’ll leave aside the practical objections about the smell and whether New Yorkers will comply. The fact is, we already have voluntary composting in New York. If the benefits are as large as Bloomberg says, why not work with, say, the Boy Scouts or other environmentally friendly groups to come up with voluntary and efficient ways to get more people to take up the practice?

Might even be nice if there were some competition here for the most efficient and creative solutions. Come to think of it, if there’s really $100 million in savings to be had, wouldn’t some clever entrepreneur be tempted to offer a solution?

Unless, of course, it’s the whole idea of government force — and the fines that go with it — that’s part of the attraction.